The present invention relates to a "multiple-stage flash-type" geothermal turbine installation in which power is derived from both high pressure steam which is separated from geothermal steam, which is a mixture of water and steam, and low pressure steam, which is obtained by flashing the remaining hot water.
A geothermal turbine installation of this general prior art type is shown in FIG. 1. The installation includes a geothermal well 1, a pipe 2 for geothermal steam which is a mixture of steam and water, a steam separator 3, a high pressure steam pipe 4 and a hot water pipe 5 extending from the steam separator, a hot water flash tank 6, a low pressure steam pipe 7 extending from the flash tank 6, a drain pipe 8 from the flash tank 6 which may in some installations be connected to a flash tank in the following stage, and a mixed-pressure turbine 9 in which a high pressure turbine section HT and a low pressure turbine section NT are formed as a single unit. The high pressure steam inlet 10 of the mixed-pressure turbine 9 is connected to the high pressure steam pipe 4 while the low pressure steam inlet 11 is connected to the low pressure steam pipe 7. The installation further includes high pressure steam inlet valves 12 and 13, a steam condenser 14 for condensing discharge steam from the mixed-pressure turbine 9, a condensation pipe 15 and a condensation pump 16 of the steam condenser 14 which as shown is formed as a jet condenser, a cooling water pipe 17 for supplying cooling water to the steam condenser 14, the gas extraction pipe 18 of the steam condenser 14, a steam ejector-type gas extractor or a mechanical gas extractor 19 such as a water ring pump, and a load 20 such as a generator which is coupled to the mixed-pressure turbine 9.
The operation of the geothermal turbine installation thus constructed will be described. The geothermal steam or the mixture of steam and water which is pumped up from the geothermal well 1 is separated into high pressure steam and hot water by the steam separator 3. The high pressure steam is introduced through the pipe 4 into the high pressure turbine section HT of the mixed-pressure turbine 9. On the other hand, the hot water is flashed by the flash tank 6 as a result of which low pressure flash steam is produced. The low pressure flash steam is introduced through the pipe 7 into the low pressure turbine section HT of the mixed-pressure turbine 9. The two types of steam are expanded in the mixed-pressure turbine 9 where they perform work and are thereafter introduced into the steam condenser 14 where they are condensed and then discharged by the condensation pump 16.
The above-described mixed-pressure type geothermal turbine is disadvantageous in the following points. The high pressure steam from the steam separator 3 is substantially saturated steam as is generally true of geothermal steam so that the humidity of steam at the turbine outlet is high. Also, high pressure steam contains a large quantity of non-condensate gas. Accordingly, because of the inevitable wear of the turbine blades in the low pressure stage and the practical capacity of the gas extractor 19, it is impossible to maintain the vacuum of the steam condenser 14 at a high level. On the other hand, the flash steam from the hot water is higher in entropy than the high pressure steam from the steam separator. Therefore, the humidity of the flash steam itself is low at the turbine outlet and the content of non-condensate gas is quite small. However, since the vacuum of the steam condenser 14 is set low depending only on the conditions of the high pressure steam, the low pressure steam is discarded wastefully with very little conversion into useful energy.
In view of the foregoing, provided in accordance with the invention is a geothermal turbine installation in which high pressure steam, which is separated from geothermal steam which is a mixture of steam and water, and low pressure steam, which is obtained by flashing the remaining hot water, are introduced into a high pressure turbine and a low pressure turbine which are formed as separate units. Discharged steam from the high pressure turbine is introduced into a steam condenser at a low vacuum, discharged steam from the low pressure turbine is introduced into a steam condenser at a high vacuum, and a water cooling system of the steam condenser of the low pressure turbine and a water cooling system of the steam condenser of the high pressure turbine are connected in series with each other. Preferably, means is provided for controlling or maintaining the ratio of the steam discharged from the high pressure turbine to the flow rate of the steam discharged from the low pressure turbine to an approximate value of 1:1.
Still further, the invention may be practiced by a geothermal turbine installation including a high pressure turbine section and a low pressure turbine section, means for separating geothermal steam into high pressure steam and hot water, means for coupling the steam from the separator to the high pressure section of the turbine, means for flashing the hot water into steam, means for coupling the steam from the flashing means to the low pressure turbine section, means for coupling a portion of high pressure steam from the high pressure turbine section after the high pressure steam has been expanded in a portion of the high pressure section to the low pressure turbine section, a low vacuum steam condenser coupled to receive steam discharged from the high pressure turbine section, and a high vacuum steam condenser coupled to receive steam discharged from the low pressure turbine section. There is preferably provided means for maintaining the ratio of the flow rate of steam discharged from the high pressure section to the flow rate of steam discharged from the low pressure section to an approximate value of 1:1.